A multi-cylinder internal combustion engine is disclosed in U.S. Pat. No. 7,152,594, which engine has a plurality of combustion chambers, fuel injectors arranged corresponding to each of the combustion chambers, and an air-fuel ratio sensor arrange for detecting an air-fuel ratio of an exhaust gas in an exhaust passage downstream of a portion where the exhaust passages each communicating with the corresponding combustion chamber converge, in which an amount of fuel injected from each fuel injector is controlled such that an air-fuel ratios of the mixtures formed in the combustion chambers are controlled to a target air-fuel ratio on the basis of the output value of the air-fuel ratio sensor.
In the above-mentioned U.S. patent, there is a description, regarding the air-fuel ratio of the exhaust gas discharged from the combustion chamber. That is, the air-fuel ratio of the exhaust gas discharged from the combustion chamber becomes great or small periodically. The air-fuel ratio of the exhaust gas changes accordingly and therefore viewing the output value of the air-fuel ratio sensor along the time axis, the line described by the output values of the air-fuel ratio sensor periodically moves up and down. Further, viewing the length of the line described by the output values of the air-fuel ratio sensor for a constant time (hereinafter, this length will be referred to as “sensor output line length”), when the air-fuel ratio of the mixture formed in the combustion chamber is accurately controlled to the target air-fuel ratio, the sensor output line length become a certain specific length in the case where the engine speed and the amount of the air sucked into the combustion chamber (hereinafter, this amount will be referred to as “intake air amount”) are both constant. On the other hand, when the air-fuel ratio of the mixture formed in one or more of the combustion chambers is not accurately controlled to the target air-fuel ratio, the sensor output line length becomes longer than the above-mentioned specific length even if the engine speed and the intake air amount are both constant.
According to the invention disclosed in the above-mentioned U.S. patent, a sensor output line length when the air-fuel ratio of the mixture formed in the combustion chamber is accurately controlled to the target air-fuel ratio is previously obtained as a base sensor output line length, depending on the engine speed and the intake air amount, the sensor output line length is calculated during the operation of the internal combustion engine, the calculated sensor output line length is compared with the base sensor output line length, depending on the engine speed and the intake air amount at that time and when the calculated sensor output line length is shorter than the base sensor output lien length, it is judged that the air-fuel ratio of the mixture formed in the combustion chamber is accurately controlled to the target air-fuel ratio. On the other hand, when the calculated sensor output line length is longer than the base sensor output line length, it is judged that the air-fuel ratio of the mixture formed in one or more of the combustion chamber is not accurately controlled, i.e. it is judged that the condition where there is a difference in the air-fuel ratio between the mixtures formed in the combustion chambers (hereinafter, this condition will be referred to as “inter-cylinder air-fuel ratio imbalance condition) occurs.
When the inter-cylinder air-fuel ratio imbalance condition occurs, the emission property of the exhaust gas discharged from the combustion chamber becomes worse. Accordingly, in the invention disclosed in the above-mentioned U.S. patent, when it is judged that the inter-cylinder air-fuel ratio imbalance condition occurs, it is indicated.
In the invention disclosed in the above-mentioned U.S. patent, as explained above, the base sensor output line length used for judging whether the inter-cylinder air-fuel ratio imbalance condition occurs, i.e. judging the occurrence of the inter-cylinder air-fuel ratio imbalance condition depends on the engine speed. Therefore, in order to judge the occurrence of the inter-cylinder air-fuel ratio imbalance condition, it is necessary to previously obtain the base sensor output line length, depending on the engine speed. However, it is a very heavy load to previously obtain the base sensor output line length, depending on the engine speed and therefore the cost of the manufacturing of the internal combustion engine increases. Therefore, the judgment of the occurrence of the inter-cylinder air-fuel ratio imbalance condition is very essential to reduce the emission of the exhaust gas, however, it is not preferred to previously obtain the base sensor output line length, depending on the engine speed in order to reduce the cost of manufacturing of the internal combustion engine. For the reasons, the technique for judging the occurrence of the inter-cylinder air-fuel ratio imbalance condition without previously obtaining the base sensor output line length, depending on the engine speed is desired. In other words, the technique for judging the occurrence of the inter-cylinder air-fuel ratio imbalance condition without previously obtaining a judgment value used for judging the occurrence of the inter-cylinder air-fuel ratio imbalance condition is desired.